The aircraft had taken off from Dublin Airport and was climbing to FL 370 enroute for Dusseldorf, when over Wallasey, the aircraft suffered a complete loss of pressurisation.The Captain initially requested ATCpermission to descend, but when this was refused due to traffic, a PAN was declared and the aircraft was immediately given clearance to descend. Meanwhile, the crew attempted to regain control of the cabin pressure using “Standby Mode” and “Manual Mode” but with no effect.As the aircraft passed FL 280, control of pressurisation was suddenly re-established.The crew decided to return to Dublin and the aircraft landed there without further incident.